The present invention is directed to difficulties in electrogalvanizing incident to overdeposition of zinc. Overdeposition is a function of the electrical characteristics of the process, specifically high current density burning. The particular variety of overdeposition to which our invention is addressed is the formation of zinc nodules, sometimes referred to as "cabbage heads" which not only are undesirable in place, but tend to become loose and may result in highly undesirable marring of the main portion of the strip and/or the finished product, such as the product of a metal stamping process. They may cause dents, dimples, and high spots on the strip. The problems of zinc pickup and the formation of cabbage heads are more pronounced where higher zinc coating weights are deposited. The cabbage heads are generally formed at the extreme edge of the strip and on the edge wall, which tend to collect high zinc coating weights.
Finding solutions for the problem is made difficult by the high production rates of typical electrogalvanizing lines. It is not uncommon for low-carbon steel strip to travel at rates of the order of 1000 feet per minute (305 meters per minute) through a series of rolls prior to entering the electrogalvanizing zone. A demanding aspect of the problem is therefore that, if an edge masking material is applied in liquid form it should be fully cured in a very few seconds (2 or 3), preferably less than one second. Other desiderata for the coating, depending on the conditions of the plating process, are that it should be tough enough to withstand mechanical abrasion from traveling at high speed through rubber and metal rolls, it should not conduct electricity, and it should be environmentally acceptable and nontoxic for ease in handling before and during use, and for disposal. In addition, the process of applying and curing the edge mask must be conveniently and continuously performed so as not to cause shutdowns or other complications. Different manufacturing facilities and processes will have different demands and specific needs, and the practitioner skilled in the art will keep them in mind when choosing a coating.
A 1968 patent to Bedi, U.S. Pat. No. 3,390,060, describes the use of two different types of waxes to protect metal during plating processes. The waxes were applied with solvents, however, which had to be dried off before the specimens could be used. Such a procedure would be wholly incompatible with a high-speed electrogalvanizing line, not only because the coating would not "set" in the very short time available for the steel strip, but also because the continuous release of solvents into the atmosphere could not be tolerated in a contemporary electrogalvanizing plant. See also Heinse U.S. Pat. No. 2,516,986, which masks stainless steel against copper plating with a wax, and Gaynes U.S. Pat. No. 2,999,771, which discusses an acid-resistant coating for use in chrome plating; the coating is a modified vinyl chloride polymer in a solvent.
Lipson et al, in U.S. Pat. No. 4,270,985, is representative of a number of disclosures of the use of radiation-curable resins as masking agents for making printed circuits, wherein the photopolymerizable resin is placed on a copper sheet or foil, for example, in the desired circuit pattern and the unprotected areas are etched away. Galvanizing may be viewed as the opposite of etching, in that metal is added to metal; the present invention is directed to a method of protecting an edge of steel strip from the deposition of zinc so as to avoid the formation of "cabbage heads"; this has nothing to do with etching away the unprotected areas of the substrate.
Levinos, in U.S. Pat. No. 3,390,061, is representative of disclosures of various solvent-based coatings used to protect areas of metal against plating by other metals, in this case to cover one side of an aluminum sheet being plated with copper. Such references merely demonstrate that coatings have been used to protect metals during plating processes. See, as an additional example, Hans' U.S. Pat. No. 4,224,118, which presents a particular resin for use as a masking agent. The present invention does not benefit from such teachings, however, because they do not deal with high speed steel strip and they generate solvent fumes. While Yoshioka et al, in U.S. Pat. No. 4,969,980, deal with high speed galvanizing, they use a protective coating for an entire surface of the strip simply to prevent the galvanizing process from plating on both sides. White et al in U.S. Pat. No. 4,587,136, describe a silicon-containing composition which is useful in our invention, saying that it could be applied to steel (col 6, line 2), but do not contemplate applicants' purpose and constraints.
The problem of edge overcoat, or excessive zinc on the edge of the finished product, is attacked by Tsuruta et al in Japanese laid-open patent 58-113396 (1983) by using one of a variety of mechanical edge masks or shields, which are shaped generally like a channel or longitudinally slit tube. In this representative patent, the edge of the strip is made to pass through the open area of the semicylindrical or U-shaped (profile) area of the edge mask or shield, thus intercepting and reducing the intensity of the electrical energy directed at the extreme edge of the strip. Such references serve to illustrate and emphasize the importance of the problem. This and other mechanical approaches are subject to many problems of maintenance and control. See, for example, U.S. Pat. No. 4,784,740 to Murakami et al, which illustrates a positioner for such mechanical devices. In Japanese patent 158386, a U-shaped profile shield is supplemented by the use of solvent-based paint on the vertical edge of the strip.
Japanese Kokai 6-158386 (1994) illustrates and describes the application of a conventional coating to the edges of steel strip in preparation for galvanizing. Its purpose is to prevent edge overcoating, but the procedure does not contemplate integration with the electrogalvanizing process, i.e. applying and curing the coating while the strip is moving at high speeds and about to enter the electrogalvanizing step.
The state of the edge-coating apparatus art is represented in Schiele's U.S. Pat. No. 5,298,072, which describes a vacuum-assisted system for moving liquid coating from a pool to the surface desired to be coated. Excess coating material is economically recirculated. The particular configuration of the edge-coating heads 3 can be changed to adapt to the requirements of various workpieces and continuous feeding mechanisms. See also Schiele's U.S. Pat. No. 5,070,080, which describes a continuous vacuum coating apparatus.
The use of ultraviolet radiation to cure efficiently liquid coatings containing photoinitiators is described by Wood in U.S. Pat. No. 4,710,638. This patent illustrates a reflector having an elliptical profile which directs the radiation from a tubular electrodeless ultraviolet source energized by microwave energy to an elongated workpiece. The principle of the elliptical reflector is that light emitted at one focus of a full ellipse will pass through the other focus. Wood's device helps assure that light emitted from a tubular source placed with its center at one of the foci of the ellipse will, as efficiently as practical, strike the workpiece occupying the other focus and its immediate surroundings.